Managing and manipulating carbocations in biology: terpenoid cyclase structure and mechanism.

Terpenoid cyclases catalyze remarkably complex cyclization cascades that are initiated by the formation of a highly reactive carbocation in a polyisoprene substrate. Recent crystal structures of terpenoid cyclases show how these enzymes provide a template for binding and stabilizing the flexible substrate in the precise orientation required for catalysis, trigger carbocation formation, chaperone the conformations of the reactive carbocation intermediates through a unique cyclization sequence, and sequester and stabilize carbocations from premature quenching. Notably, terpenoid cyclases and catalytic antibodies have converged to similar chemical and structural strategies for managing highly reactive carbocations in polyisoprene cyclization cascades.